Environmental Impact

Here is a four-part series by the Michigan Wildlife Conservancy on the on problems that can result when there are too many deer. The first of the series gives an overview of the issues. The remaining three articles deal with impacts on specific kinds of plants and animals, and methods to control deer numbers.

WHITE-TAILED DEER IN THE HUDSON VALLEY:History, ecology, and impacts of a keystone species, News from Hudsonia, Fall 2018White-tailed deer are recognized as “key-stone” herbivores that profoundly affect forest structure and succession; when over-abundant, they tend to reduce the numbers of species and individuals of native forest plants and increase the proportion of non-native plants. Where deer abundance exceeds15/mi, tree seedling abundance is reduced in many forest types in the northern US.
Overabundant deer also affect breeding bird communities, invertebrates that depend on understory plants, squirrel populations (which inturn affect bird nesting success), and tick abundance and the prevalence of tick-borne diseases.

Hungry Deer May Be Changing How Things Sound In The Forest, NPR, Feb 13, 2019“The deer are very, very over abundant,” says Megan Gall, an ecologist at Vassar College who studies how the environment shapes animals’ senses. “It’s much lusher when there are fewer deer around, and so that’s a big change in the structure of the environment.”
She got intrigued by the possibility that deer might affect the soundscape after talking with a couple of colleagues who were studying how browsing deer could transform a forest ecosystem by munching through the entire lower level of leafy plants. “When deer were browsing, we actually found that the sound was clearer,” Gall says, “and that’s probably because there was less vegetation in the way. You don’t have as many sounds bouncing off of leaves and sticks and things like that. When deer are browsing, the sound actually has higher fidelity as it moves through environment.”

New study finds zoning ineffective for deer winter habitat conservation, Phys.org, Aug 24, 2018“Our results suggest that northern Maine is losing the potential for future replacement of viable areas for wintering deer,” the researchers say. The substantial rates of loss and fragmentation documented show that “habitat conservation strategies that rely on reserves and ignore land use effects on the intervening lands may not be effective.”Weeds, worms, and deer: positive relationships among common forest understory stressors, Biological Invasions, May 201828% of plots had a combination of earthworms absent, low deer browse, and no non-native plants, and 29% of plots included earthworms, non-native plants, and moderate or greater browse damage. Through SEM, we found strong support for pathways among global change stressors, e.g., deer browse positively influenced earthworm presence and both deer and earthworms promoted non-native plants. Warmer air temperatures and higher soil pH also facilitated non-natives. This research highlights the tremendous multipronged management challenge for areas already experiencing the combined effects of weeds, worms, and deer and the future vulnerability of other areas as temperatures warm and conditions become more amenable to biotic global change stressors.

Without its ‘understory’ layer, the forest will collapse, New Jersey Herald, Feb 23, 2018According to Dr. Jay Kelly, a biology professor at Raritan Valley Community College who is studying forest health, many forests are in trouble because their lower understory layers are disappearing. “They’re being decimated by deer and invasive plants,” he explains. The number of medium and large trees has decreased only slightly since Buell’s time, but saplings have plummeted by 85 percent and small trees by 90 percent. “If this trend continues, we’re actually going to be losing forest as the older trees die, because there are no new trees to replace them,” Kelly said.Special hunt gets 47 deer on Lake Michigan island once eyed as sportsman’s preserve, MLink, Nov 21, 2017In the 1920’s, nine deer were brought to North Manitou Island in Sleeping Bear Dunes National Lakeshore. “The deer thrived and rapidly multiplied and were allowed to roam across the entire island. The forest began to have the groomed appearance of a city park as a result of the deer feeding on ground cover and low-hanging branches. They seemed to favor maple and juniper, but eventually browsed on everything except beech, which continues to spread across the island,” according to the island’s history page on the park’s website.

Natural Lands staff track impact deer have on ecosystem, The Ithacan, Nov 9, 2017According to Cornell’s Botanical Gardens website, they control the deer population to preserve and increase the well-being of the forest. The implementation of this form of deer control depends on factors like what the land is used for and if firearms are permitted. Todd Bittner, director of Cornell University Natural Areas, said. Bittner said that assessing how Lyme disease occurrences relate to the population of deer is a key interest in Cornell’s program, but more funding is necessary for Cornell’s study. Cornell Health at Cornell University said they were unable to release the information of the number of Lyme disease cases. However, at Ithaca College over the past five years, Hammond Health Center has had 41 patients diagnosed with Lyme disease and 79 patients with tick bites, Vivian Lorenzo, director of medical services, said.

Are Native Tree Seedlings Facilitated by an Invasive Shrub Where White-Tailed Deer Are Abundant?, BioOne, Oct 1, 2017Positive interactions (facilitation) between plant species have been documented, particularly in stressful environments. We investigated whether an invasive shrub enhances growth or survival of native tree seedlings in forests where white-tailed deer (Odocoileus virginianus) are abundant. Seedlings of four tree species were planted under, or 0.5 m outside of, the canopy of Amur honeysuckle (Lonicera maackii) shrubs in plots unfenced or fenced to exclude deer in two stands in southwestern Ohio, USA, and monitored for 1 y. One species succumbed to transplant shock, but browse was extensive on the other three species in unfenced plots. Facilitation was evident on sugar maple (Acer saccharum), as seedlings under shrub cover had higher survival and final leaf count in unfenced plots, but not where deer were excluded. However, there was no significant facilitation by L. maackii of seedling growth or survival for white oak (Quercus alba) or shagbark hickory (Carya ovata), indicating this shrub’s branches do not deter deer browse on highly preferred species or the effect is too modest to emerge in a one-year study. In at least some cases, the negative effects of L. maackii on tree seedlings that have been documented previously are offset by mitigation of the negative effects of deer herbivory. This suggests that where browsing impacts are problematic, deer abundance should be reduced before this, and perhaps other invasive shrubs, are managed.

Assessing Forest Health in Central New Jersey: Impacts of Deer and Invasive Plant Species, 9th Annual Sustainable Raritan Conference, June 9, 2017It’s clear to Kelly that some combination of these deer control measures is needed. If the number of deer per square mile isn’t reduced, he said, thousands of forest plants and animals will not survive. Kelly said that humans also benefit from fewer deer. Towns with reduced deer populations have fewer deer-automobile collisions and lower rates of tick-borne illnesses like Lyme disease.

Plans for habitat and wildlife conservation need to consider the risk of Lyme disease, University of Glasgow, May 9, 2017Lead author Dr Caroline Millins, from the University of Glasgow’s School of Veterinary Medicine and Institute of Biodiversity, Animal Health and Comparative Medicine (BAHCM), said: “We identified several widespread conservation management practices which could affect Lyme disease risk: the management of deer populations, woodland regeneration, urban greening and control of invasive species.

Effects of conservation management of landscapes and vertebrate communities on Lyme borreliosis risk in the United Kingdom, Transactions of the Royal Society, April 24, 2017Deer can feed large numbers of adult female ticks, which then lay eggs and produce the next generation of immature ticks, and deer are thus termed ‘tick reproduction hosts’. A great many studies have shown that deer can be instrumental in maintaining tick populations, such that areas with more deer also have more ticks although there is some uncertainty in the precise relationship between deer density and tick density. Some of these studies specifically tested the impact of deer management methods and, when deer numbers were reduced through culling or fencing, there were dramatic declines in the tick population.

Wisconsin Public Radio reports that biologists have known for a long time that an overabundance of deer negatively affects the number and diversity of plants in our forests. But a new study headed by Autumn Sabo, a PhD candidate at the university, suggests it’s also changing the soil beneath the forest floor. Sabo took samples from test plots that have been fenced off from deer for up to two decades and found less soil compaction as well as a thinner layer of depleted soil, which is called a leach zone.

The Future of Oak Forests: Ecology, Management, and Regeneration, Ohio State University Extension, Jan 2017

Rhode Island’s Large Deer Population More Immediate Threat to State’s Forests Than Climate Change, RI eco, Dec 1, 2016Rhode Island’s forests are already facing what some say is an even greater threat than climate change: an overabundance of deer. That’s the warning from foresters, biologists and ecologists from throughout the Northeast, who say that even without climate change, Rhode Island’s forests are in trouble unless the state’s deer herd can be reduced and managed more effectively.“They’ve browsed all of the favorable species like oaks and maples, they’ve destroyed our wildflowers, and a lot of the understory plants they like to eat are the ones we rely on for the future stocking of the forest,” Tremblay said. “What’s worse, they don’t like invasive species, so barberry and buckthorn and other invasives are growing like crazy. The end result is a complete alteration of the forest, where the invasives have a leg up.”

Deer herbivory reduces web-building spider abundance by simplifying forest vegetation structure, PeerJ, Sept 2016Indirect ecological effects are a common feature of ecological systems, arising when one species affects interactions among two or more other species. We examined how browsing by white-tailed deer indirectly affected the abundance and composition of a web-building spider guild through their effects on the structure of the ground and shrub layers of northern hardwood forests. We examined paired plots consisting of deer-free and control plots in the Allegheny Plateau region Pennsylvania and Northern Highlands region of Wisconsin. We recorded the abundance of seven types of webs, each corresponding to a family of web-building spiders. We quantified vegetation structure and habitat suitability for the spiders by computing a web scaffold availability index (WSAI) at 0.5 m and 1.0 m above the ground. At Northern Highlands sites, we recorded prey availability. Spider webs were twice as abundant in deer-free plots compared to control plots, while WSAI was 7-12 times greater in deerfree plots. Prey availability was lower in deer-free plots. With the exception of funnel web-builders, all spider web types were significantly more abundant in deer-free plots. Both deer exclusion and the geographic region of plots were significant predictors of spider community structure. In closed canopy forests with high browsing pressure, the low density of tree saplings and shrubs provides few locations for web-building spiders to anchor webs. Recruitment of these spiders may become coupled with forest disturbance events that increase tree and shrub recruitment. By modifying habitat structure, deer appear to indirectly modify arthropod food web interactions. As deer populations have increased in eastern North America over the past several decades, the effects of deer on web-building spiders may be widespread.

Of wolves, deer, maples and wildflowers, Great Lakes Echo, June 26, 2016Plant-eaters can have a major impact on environmental change, including biodiversity and the structure of plant communities, the study said, and the findings may help managers of wildlife and public lands in the Great Lakes region. Differences were dramatic. Deer density was 62 percent lower in high-wolf areas, where deer visits were 82 percent lower and foraging time was 43 percent shorter, it said.

Impacts of deer on northeastern forests and strategies for control, Paul Curtis, Cornell University, June 2016

Deer-Forest Study, Penn State, Dept of Ecosystem Science and ManagementPennsylvania forests face many challenges – invasive plants, insect outbreaks, soil acidity, tree diseases, and even deer. This study is being conducted to better understand the role of deer in the context of all these challenges and to help Pennsylvania’s forest and wildlife managers better manage deer and the forest.

When the Killing’s Done, Island Wildlife Roars Back, Takeapart, March 21, 2016New research shows that eradicating invasive species lets unique and imperiled wildlife recover. Jones, who teaches at Northern Illinois University, is the lead author on a study published Monday in Proceedings of the National Academy of Sciences that looks at the long-term effects of eradicating cats, rats, goats, pigs, and other invasive mammals from islands. On the 181 islands where biologists have conducted follow-up studies, Jones and her coauthors found that eradication turns out to be one of the most effective strategies “for protecting the world’s most threatened species.”

I have discussed the urban deer issue with academic biologists at the University of Michigan, including ecologists, botanists, zoologists, restoration ecologists and landscape architects. We are all in support of city council’s decision to conduct a cull.

From ecological and conservation perspectives, an ideal deer herd will coexist with a full range of native species. By several measures, Ann Arbor’s herd size has surpassed this threshold. Botanists at the U-M have long noted declines in native plants that deer favor, through decades of observation, and by comparison with landscapes where deer are excluded or managed. In a 2015 study, an ecological team surveyed browsing impacts in Ann Arbor’s Bird Hills Nature Area. They found browsing damage in 80 percent of the tree saplings.

Because of the importance of healthy forests to people and the whole forest ecosystem, DCNR has long advocated balancing deer populations using habitat quality as the primary measure. That is, the health of the forest understory and the forest’s capacity to renew itself.

Deer Can Be Too Many, Too Few, or Just Enough Too Few, or Just Enough for Healthy Forest, US Forest Service, 2012In some places, deer populations are now so high that they cause long-term negative ecological effects, eating out forest understories of wildflowers, shrubs, and tree seedlings. Such forests are pretty obvious—consisting only of tree trunks and a few deer-resistant shrub species. Often, the only understory plant species are native New York and hay-scented ferns and nonnative invasive plants such as barberry, burning bush, and garlic mustard. In areas with high deer densities and little deer forage, the forest may not regrow at all after disturbance, leaving “fern deserts.”

Is HSUS Bad for the Environment? HumaneWatch.org, April 9, 2014Animal rights groups ideologically opposed to hunting, such as the Humane Society of the United States, have proposed deer “birth control” as a “humane” means of dealing with deer overpopulation. Cambrone, however, points out the impracticality of HSUS’s preferred (and seemingly only) way of managing deer:

Some environmentalists, especially in urban areas, oppose hunting to cull the herds and argue instead for deer “birth control.” Yet contrary to persistent urban legend, there’s no handy oral deer contraceptive we can slip into a pile of acorns. Nor is there a permanent contraceptive that can be delivered with a single shot from a dart gun. Currently available immunocontraceptive agents have no effect on 10%-15% of the treated does. Even when they’re effective the first year, booster shots are needed in subsequent years.

Then, too, it’s difficult to inject enough does in a large, free-roaming population—and more difficult still to inject each one repeatedly, right on schedule. Even if we could, all those deer would still be present for years—still eating, still wandering out into traffic, and every day welcoming their fertile new friends arriving from nearby. The most optimistic cost estimates for each injection are around $500 per deer. Even surgical sterilization has been tried in a few locales. Although it costs over $1,000 per deer, it is 100% reliable and permanent.

Changing Landscapes for White-Tailed Deer Management in the 21st Century: Parcelization of Land Ownership and Evolving Stakeholder Values in Michigan, Wildlife Society Bulletin, Sept 2011 To address this challenge, we 1) quantified how landscape characteristics in southwestern and south-central Michigan, USA affect the distribution and population characteristics of deer throughout agro-forested landscapes, 2) quantified factors affecting stakeholder acceptance capacity for deer, and 3) present a deer management framework based on desired levels of impacts in relation to existing conditions. Knowledge of deer ecology, landscape characteristics, and responses of stakeholders to deer are critical for managing the impacts of white-tailed deer.

Legacy of top-down herbivore pressure ricochets back up multiple trophic levels in forest canopies over 30 years, Ecosphere, Jan 2011Removal of top-down control on herbivores can result in a trophic cascade where herbivore pressure on plants results in changes in plant communities. These altered plant communities are hypothesized to exert bottom-up control on subsequent herbivory via changes in plant quality or productivity. But it remains untested whether top-down perturbation causes long term changes in plants that ricochet back up the new food chain that depends on them. In a large-scale, 30-yr controlled field experiment, we show that 10 yr of top-down control of an ungulate herbivore (white-tailed deer, Odocoileus virginianus) created contrasting forest tree communities exerting bottom-up effects that ricochet back up 3 trophic levels 20–30 yr later. Higher ungulate densities during stand initiation caused significant reductions in tree species diversity, canopy foliage density, canopy insect density, and bird density in young (ca. 30 yr old) forests. Because recruitment of trees from seedlings to the canopy occurs over a relatively brief period (ca. 10 yr), with membership in the canopy lasting an order of magnitude longer, our results show that even short-term perturbations in ungulate density may cause centuries-long disruptions to forest ecosystem structure and function. In documenting this five-step trophic ricochet, we unite key concepts of trophic theory with the extensive literature on effects of ungulate overabundance. As predators decline and ungulate herbivores increase worldwide, similar impacts may result that persist long after herbivore density becomes effectively managed.

History of Deer Population in Indiana, 2010, Presentation by IU Biologists Clay & SheltonTrophic cascades are often drastically disrupted by human interventions— for example,
when wolves and cougars are removed, allowing deer and beaver to become destructive— yet have only recently begun to be considered in the development of conservation and management strategies.

Matthaei Botanical Gardens & Nichols Arboretum, 2009 Fiscal Year Annual Report, UMWe’re also fortunate that our lands encompass much of the natural heritage that once was southeastern Michigan—oak woodlands, floodplain woods, tamarack swamps, savannas or oak openings,
prairies, fens, and bogs. We have significant frontage on the Huron River as well as a major stretch of Fleming Creek, one of the healthiest streams in our area. While it may seem paradoxical, these sites must be carefully managed to preserve their “naturalness” for future generations, as their lands and waters battle constant threats from invasive species, over-population of deer, and changes in hydrology.

F R I E N D S for friends of the University of Michigan’s Matthaei Botanical Gardens and Nichols Arboretum, Spring 2006Come take a walk past a woodland gem, Kirk Woods, located next to the Demonstration Prairie at the northern portion of the Gardens. It is remnant oak-opening woodland. Oak-opening
ecosystems are few and far between in Southeast Michigan. With fire suppression and deer browse leading to a predominance of fire susceptible species like maple, this type of forest is fast becoming a thing of the past. Oak-opening ecosystems were once very common in SE Michigan and were perpetuated by the Native American cultural practice of burning their land. This custom helped to stimulate certain plants used for food and other purposes, aided in hunting, and provided open views of neighboring tribes and incoming settlers.

Annual estimates of deer damage are reported to exceed $2 billion nationwide, including $1 billion in car damages, more than $100 million in agricultural crop damage, $750 million in damage to the timber industry, and more than $250 million in damage to metropolitan households (e.g., landscape plantings). These estimates are conservative, and it is often difficult to obtain reliable statistics for wildlife-related losses.

Forester Perceptions of Deer Depredation On the Forests of Michigan, Michigan Society of American Foresters, January 2008The responses from foresters clearly indicate that, in their professional opinions, negative ecological impacts of deer depredation are common across wide portions of Michigan’s forests. While this survey assessed the perspective of foresters regarding deer depredation, the collective experience of foresters is extensive.

Are we helpless?

"The native plants are tramped down, the bushes are gnawed, and my three-year-old grandson can't play in the back yard because of the deer droppings. If humans entered our property and exacted such a toll we would have legal recourse We're watching the curb appeal and property value decline at a time when our taxes are rising. We are without defense."
M. Holland, Ann Arbor resident